Electrode-Supported Solid State Electrochemical Cell

ABSTRACT

A process for manufacturing a solid oxide fuel cell involves forming a tubular anode comprising an electrolyte substance and an oxide of an electrochemically active metallic substance without a distinct pore forming substance, sintering the tubular anode, forming an electrolyte onto the sintered anode, forming a cathode onto the electrolyte, and after forming the electrolyte and the cathode, reducing the oxide of the electrochemically active substance in the sintered anode to form pores in the anode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and therefore claims priority from,U.S. patent application Ser. No. 09/864,070 filed May 22, 2001, issuingas U.S. Pat. No. 7,416,802 on Aug. 26, 2008, which claims priority fromU.S. Provisional Patent Application No. 60/206,456 filed May 22, 2000,each of which is hereby incorporated herein by reference in itsentirety.

TECHNICAL FIELD

This invention relates to solid state electrochemical cells; and inparticular to fuel cells, oxygen sensors, and oxygen pumps, and methodsof manufacture.

BACKGROUND OF THE INVENTION

Worldwide forecasts show electricity consumption increasing dramaticallyin the next decades, largely due to economic growth in developingcountries that lack national power grids. This increased consumption,together with the deregulation of electrical utilities in industrializednations, creates the need for small scale, distributed generation ofelectricity.

Fuel cells are a promising technology for providing distributedgeneration of electricity. A fuel cell places an oxidizing gas, such asair, and a hydrogen-containing fuel, such as hydrogen or natural gas, onopposite sides of an electrolyte in such a way that they combine to formwater and electricity. Such a reaction requires a cathode and an anodecomposed of porous materials, and an ionically-conducting electrolyte.In solid oxide fuel cells, the electrolyte conducts negatively-chargedoxygen ions.

Solid oxide fuel cell systems can be made less expensively than otherkinds of fuel cells, and thus have particular potential for facilitatingdistributed power generation.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided amethod for manufacturing a solid oxide fuel cell. The method involvesforming a tubular anode comprising an electrolyte substance and an oxideof an electrochemically active metallic substance without a distinctpore forming substance; sintering the tubular anode; forming anelectrolyte onto the sintered anode; forming a cathode onto theelectrolyte; and after forming the electrolyte and the cathode, reducingthe oxide of the electrochemically active substance in the sinteredanode to form pores in the anode.

In an alternative embodiment, sintering the tubular anode may involvedrying the tubular anode and sintering the tubular anode in air in afurnace having a furnace temperature ramp rate of approximately 0.5° C.per minute, up to approximately 500° C., followed by a ramp rate ofapproximately 3° C. per minute up to approximately 1300° C., and a dwelltime of approximately 2 hours for sintering.

The electrolyte substance of the anode may include stabilized zirconiaand the oxide of the electrochemically active metallic substance mayinclude a nickel oxide, a platinum oxide, a palladium oxide, or a cobaltoxide.

Forming and sintering the tubular anode may involve forming a plasticmass comprising a mixture of the electrolyte substance and the oxide ofthe electrochemically active metallic substance, extruding the plasticmass through a die to form an extruded tube, and sintering the extrudedtube.

The anode may include two or more anode layers, each layer having adifferent composition. In such embodiments, each of the anode layerstypically includes a ratio of electrochemically active metallicsubstance to electrolyte substance, and such ratios may be higher forlayers that are layered further from a surface of the anode thatcontacts a fuel gas than for layers that are layered closer to the fuelgas.

In embodiments having two or more anode layers, forming and sinteringthe tubular anode may involve forming first and second plastic masses,each plastic mass comprising a mixture of the electrolyte substance andthe oxide of the electrochemically active metallic substance, the firstplastic mass having a higher relative content ratio of oxide toelectrolyte substance, and the second plastic mass having a lowerrelative content ratio of oxide to electrolyte substance; extruding thefirst plastic mass through a die to form a first extruded tube;extruding the second plastic mass through a die to form a secondextruded tube; fitting the first extruded tube inside the secondextruded tube to form a combined tube; and sintering the combined tube.Each plastic mass may include a mixture of stabilized zirconia andnickel oxide, the first plastic mass having a higher relative contentratio of nickel oxide to stabilized zirconia, and the second plasticmass having a lower relative content ratio of nickel oxide to stabilizedzirconia. Alternatively, forming and sintering the tubular anode mayinvolve co-extruding more than one anode layer to form a co-extrudedtube; and sintering the co-extruded tube.

The two or more anode layers may include a thicker support layer forcontact with a fuel gas and a thinner active layer. The support layermay include a higher ratio of stabilized zirconia to nickel, and theactive layer may include a lower such ratio. For example, the supportlayer may include from 0% to 50% nickel by volume, while the activelayer may include from 40% to 45% nickel by volume. The active layer mayinclude an embedded current-collecting wire. The active layer may beextruded around the current-collecting wire. The support layer mayinclude aluminum oxide.

The anode may include a catalyst material, such as CeO₂, ruthenium,rhodium, rhenium, palladium, scandia, titania, vanadia, chromium,manganese, iron, cobalt, nickel, zinc, or copper. For example, thecatalyst material may include CeO₂ in a proportion of between 1% and 3%by weight. Forming the anode may include milling the catalyst with theoxide of the electrochemically active substance.

Reducing the oxide of the electrochemically active metallic substance inthe sintered anode to form pores may involve flowing a reducing gas overa surface of the sintered anode. For example, reducing the oxide of theelectrochemically active metallic substance in the sintered anode toform pores may involve flowing hydrogen gas over a surface of thesintered anode at a temperature between approximately 800° C. and 1000°C.

The anode may include a substantially uniform ratio of electrochemicallyactive metallic substance to electrolyte substance. The anode mayinclude a volume percentage of nickel of between 40% and 50%. Athickness of the anode may be over 50% of a total thickness of theanode, the electrolyte, and the cathode. The anode may have a thicknessof at least 300 μm. The anode may have a circular or non-circularcross-section.

The electrolyte formed onto the sintered anode may include stabilizedzirconia and may be formed by spraying or dip-coating.

The cathode may include a strontia-doped lanthanum manganite, gadoliniummanganate, or a cobaltate and may be formed by spraying. The cathode mayinclude two or more cathode layers having different compositions. Forexample, a cathode having two cathode layers may include an innercathode layer comprising a mixture, 50/50 wt % of La_(0.80)Sr_(0.20)MnO₃(Rhodia, 99.9% pure) with 8 mol % YSZ (Tosoh) and an outer cathode layercomprising substantially only La_(0.80)Sr_(0.20)MnO₃ (Rhodia, 99.9%pure).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show cross-sectional isometric and cross-sectionalviews, respectively, of an electrolyte-supported tubular solid oxidefuel cell;

FIGS. 2A and 2B show cross-sectional isometric and cross-sectionalviews, respectively, of an anode-supported fuel cell according to anembodiment of the invention;

FIG. 3 shows a block diagram of a process for manufacturing a tubularsolid oxide fuel cell in accordance with an embodiment of the invention;

FIG. 4 shows a graph of chemical gradients in a co-extruded anodesupport of a fuel cell, according to an embodiment of the invention;

FIG. 5 shows a block diagram of a process for manufacturing a solidoxide fuel cell that includes co-extruding a cell-supporting anode, inaccordance with an embodiment of the invention;

FIGS. 6A and 6B show an anode-supported tubular solid oxide fuel cell,in accordance with an embodiment of the invention, that may bemanufactured by the co-extrusion process of FIG. 5; and

FIGS. 7A and 7B show an electrode-supported oxygen pump or oxygen sensoraccording to an embodiment of the invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Embodiments of the present invention allow the production of solid stateelectrochemical cells that have a lower electrical resistance than suchsystems have had in the past; that are less expensive; and that achievefaster response times.

FIGS. 1A and 1B show cross-sectional isometric and cross-sectionalviews, respectively, of an electrolyte-supported tubular solid oxidefuel cell 100. Tubular cell 100 is formed by three concentric tubularlayers 120, 140, and 160. The middle, electrolyte layer 140 performs twofunctions: first, it provides mechanical support for cell 100; andsecond, it provides an ionic conduction pathway for negatively-chargedoxygen ions. These oxygen ions are produced by an oxidizing gas 125(such as oxygen, present in air) that surrounds the tube's outerdiameter. The electrolyte conducts the oxygen ions from the outer,cathode layer 120 to the inner, anode layer 160, each of which layers isformed from a porous substance. A source of hydrogen 165, such ashydrogen gas (H₂) or natural gas, is passed through the hollow center ofthe cell 100 and reacts with the oxygen ions to produce water andelectricity. The cathode 120 and anode 160 are connected to terminals(not shown) for conducting electrical current to and from the cell.

In cell 100, electrolyte 140 is formed from a ceramic such asyttria-doped stabilized zirconia (YSZ). Electrolyte 140 has a thicknessT₁ of about 200 μm that enables it to provide mechanical support forcell 100; the cell is therefore an electrolyte-supported cell. Once theelectrolyte has been formed, cathode 120 is typically sprayed onto theoutside of electrolyte 140 to a thickness T₂ of about 50 μm to 100 μm;and anode 160 is sprayed onto the inside of electrolyte 140 to athickness T₃ of about 50 μm.

A disadvantage of cell 100 is that electrolyte 140 must be thick enoughto support the cell mechanically. According to Ohm's law, the resistanceacross an electrolyte of thickness l, resistivity ρ, and cross-sectionalarea A is given by:

$\begin{matrix}{R = \frac{\rho \; l}{A}} & \left\{ {{Equation}\mspace{14mu} 1} \right\}\end{matrix}$

It follows from Equation 1 that an electrolyte of greater thickness (l)has a higher electrical resistance (R). Since, in cell 100, electrolyte140 must be thick enough to support the cell, it has a high value of land thus a greater resistance. Since power losses are proportional toresistance, more cells 100 must be used, to produce a given poweroutput, than would have to be used if cell 100 had a lower resistance.Thus fuel cell systems using such cells are more expensive than theywould be otherwise. Also, since the fuel cell reaction occurs at hightemperature, the start-up time of a fuel cell system is limited by thetime that it takes for the cells to heat up to the reaction temperature.Systems containing more cells for a given power output require longer toheat up, and thus longer to start. Systems containingelectrolyte-supported cells 100 are thus larger, more expensive, andslower to start than they would be if cell 100 had a lower resistance.

Attempts have been made to solve this problem by using materials forelectrolyte 140 that have a lower resistivity ρ (see Equation 1).Materials containing CeO₂, Bi₂O₃, and LaGaO₃ (along with secondarydopants) have produced dramatic reductions in resistivity ρ. However,these materials have disadvantages in other characteristics. CeO₂reduces easily to CeO_(2(-x)); Bi₂O₃ melts at low temperatures and isextremely volatile; LaGaO₃ suffers from evaporation of Ga; and all threeare mechanically weak. Most notably, however, all three are presentlyexpensive: LaGaO₃ costs about 1000 times as much as yttria-stabilizedzirconia (YSZ).

Another fuel cell design is found in U.S. Pat. No. 5,998,056 of Diviseket al., which discloses an anode substrate for a planar fuel cell.Planar fuel cells suffer from several disadvantages by comparison withtubular fuel cells. In particular, because of their planar shape, theyare difficult to seal to prevent gas leaks.

Other designs use cathode-supported tubular fuel cells. These, however,are relatively expensive.

The disadvantages of these designs are overcome by the anode-supportedtubular fuel cell shown in FIGS. 2A (cross-sectional isometric view) and2B (cross-sectional view), according to an embodiment of the invention.In this embodiment, tubular solid oxide fuel cell 200 is mechanicallysupported by a thick anode layer 260, upon which thin electrolyte layer240 and cathode layer 220 are formed. Since fuel cell 200 ismechanically supported by anode 260, there is no need for a thickenedelectrolyte layer to support the cell, and the electrolyte layer 240 maybe reduced in thickness (l) as compared with the thickness of theelectrolyte of an electrolyte-supported fuel cell. As follows fromEquation 1, a lower thickness (l) means that fuel cell 200 has a lowerresistance. For the reasons described above, a fuel cell system that canincorporate such cells, which have a lower resistance, is lessexpensive, more compact, and faster to start than electrolyte-supportedsystems. It also avoids the expense of cathode-supported fuel cells.Additionally, it can avoid disadvantages with sealing found in planarsystems, because tubular fuel cells according to embodiments of theinvention may be mounted in holes in a base plate, and used with theirends open (though open ends are not required—embodiments may also have aclosed end on the fuel cell). Thus, anode-supported fuel cells inaccordance with embodiments of the invention provide advantages overconventional designs, in both cost and performance, that areimprovements for the distributed generation of electricity.

Fuel cell 200, according to an embodiment of the invention, reactsnegative oxygen ions from an oxidizing gas 225 (such as the oxygen foundin air) with hydrogen found in fuel 265 (which may be hydrogen gas,natural gas, a hydrocarbon gas, or another source of hydrogen). Oxygenions pass through porous cathode layer 220 and are ionically conductedthrough electrolyte layer 240 to the porous cell-supporting anode layer260, where they react with the fuel 265 to form electricity and water.Terminals (not shown) conduct electrical current to and from the cell.

As can be seen in FIG. 2B, thickness T₁ of electrolyte layer 240 isgreatly reduced by comparison with thickness T₃ of anode layer 260,since electrolyte layer 240 does not need to provide mechanical supportto the cell. The thickness of the anode layer, according to embodimentsof the invention, may comprise 50% or more of the total thickness of thefuel cell wall (T₁ plus T₂ plus T₃), and the electrolyte layer may becorrespondingly reduced in thickness.

FIG. 3 is a block diagram that summarizes a process for manufacturing atubular solid oxide fuel cell in accordance with an embodiment of theinvention. The solid ingredients of the cell's anode preferably includenickel oxide (NiO), which provides the electrochemical activity for theanode, and yttria-doped stabilized zirconia (YSZ) as an electrolytesubstance. Other metals, instead of nickel, may provide electrochemicalactivity; for example, platinum, palladium, and cobalt may be used inaccordance with embodiments of the invention. Also, other electrolytesubstances may be used. In a preferred embodiment, the anode is made ofa 50 vol % mixture of NiO and 8 mol % YSZ. In addition to anelectrochemically active substance and an electrolyte substance, theanode is preferably fabricated using a binder system, a lubricant, a pHcontrol agent, and solvents. In a preferred embodiment the binder systemis the Duramax™ binder system, B-1051 and B1052, manufactured at thetime of filing by Rohm and Haas Co. of Philadelphia, Pa.; the lubricantis PEG-400, manufactured at the time of filing by Union Carbide ofDanbury, Conn.; the pH control agent is AMP-95, manufactured at the timeof filing by Angis Chemicals of Buffalo Grove, Ill.; and the solventsare acetone, distilled water, or isopropyl alcohol.

In describing the process of the embodiment of FIG. 3, a specificembodiment of a process is described; those of ordinary skill in the artwill recognize, however, that variations from this process are possiblein accordance with embodiments of the invention.

In step 301 of the process of the embodiment of FIG. 3, NiO powder ismilled in a solvent (preferably isopropyl alcohol) by roller mill in 250ml Nalgene containers with 10 mm diameter zirconia grinding media, for48 hours. In step 302, YSZ powder is heat treated at 900° C. for 2 hoursto reduce the powder surface area. In step 303, the YSZ powder is addedto the NiO powder and milled a further 24 hours. The milled powders arenext dried by evaporating off the solvent in an oven at 75° C. for 12hours, in step 304. A particle size analysis is performed on theresulting powders using a laser diffraction particle sizer (such as onesold at the time of filing as the Malvern Mastersizer) to determine theeffect of the solvent on particle size, and the distribution of particlesizes. In step 305, the powders are hand ground and mixed with theadditives—binders, lubricants, pH control agents, and solvents (here,preferably water)—to produce pastes 306; other additives may includeplasticizers, deflocculants, catalysts, and other ingredients. Thepastes are next aged overnight in plastic bags to produce extrudablepastes (step 307).

Next, in step 308 of the embodiment of FIG. 3, the extrudable pastes areextruded through a die into a tube or rod form 309, using a highstrength steel piston extruder coupled to a tensile testing machine(such as a Lloyd LR100K tensile testing machine). The piston speed iscontrolled by a personal computer, and preferably set at a rate of 3 mmper minute. Following extrusion, the extruded tubes are driedhorizontally on a V-shaped alumina sample holder for 24 hours and thensintered in air in a chamber furnace (such as a furnace manufactured atthe time of filing by Ceramic Engineering) (step 310). A furnacetemperature ramp rate of 0.5° C. per minute, up to 500° C., is used toburn off the organic substances; this is followed by a ramp rate of 3°C. per minute up to 1300° C., and a dwell time of 2 hours for sintering.The result is a set of anode tubes (one made from each extruded tube)each capable of supporting a fuel cell; in one embodiment, an anode hasa thickness in the range of 300 μm to 400 μm.

Next, in step 311, an electrolyte layer is coated onto the anode tube.The electrolyte layer may be formed of yttria-doped stabilized zirconia(YSZ), and may be sprayed, dip-coated, or otherwise layered onto theanode tube. Preferably, a YSZ slurry is prepared using the electrode inkmethodology, and a thin coating is then sprayed onto the sintered anodesupport tube to form an electrolyte layer. The electrolyte layer is thendried in air and isostatically pressed at 200 MPa. Finally theelectrolyte layer is sintered at 1350° C. for 2 hours to form a fullydense membrane of about 20 μm thickness.

Next, in step 312, a cathode layer is coated onto the electrolyte layer.The cathode layer is preferably made from strontia-doped lanthanummanganite (LaMnO₃), but may also be made from gadolinium manganate, acobaltate, or other substances. In a preferred embodiment, two cathodelayers are applied to the outside of the electrolyte layer by using aspray gun to form thin, even cathode layers on the electrolyte layer'ssurface. The first (inner) cathode layer is preferably a mixture, 50/50wt % of La_(0.80)Sr_(0.20)MnO₃ (Rhodia, 99.9% pure) with 8 mol % YSZ(Tosoh). The second cathode layer is preferably onlyLa_(0.80)Sr_(0.20)MnO₃ (Rhodia, 99.9% pure).

Finally, in step 313, current collectors are connected to the anode andcathode layers to complete the fuel cell's fabrication. The cathodecurrent collector is preferably made of silver wire (Alfa 99.997% pure)of 0.25 mm diameter, and wound crisscross along the anode length withclose contact between windings. Silver paste (Alfa) is preferablypainted onto the cathode and air dried, before the current collection iswound onto it. The anode current collector is preferably made of nickelwire (Alfa 99.98% pure) of 0.5 mm diameter, and is spiraled around a 1mm diameter needle former to produce a tight coil. The coil is fed intothe fuel cell by jamming inside the tube, to produce a good contact.

An example of preparation of cell-supporting anode tubes according to anembodiment of the invention is now provided. It should be recognizedthat this example is provided for the purpose of illustration, andshould not be taken to limit the invention to the example given.

Example

65.08 g of NiO powder was milled in 100 g isopropanol in a 1 L plasticmilling container with 1 kg of 5 mm diameter milling media, at 25 Hz,for 42-48 hours, until a particle size of 0.8 μm was reached. 7.19 g of8 mol % yttria stabilized zirconia (8YSZ) (which had been calcined for 2hours at 900° C.), and 27.72 g 8YSZ (which had been calcined for 2 hoursat 1100° C.), were added to the milling container and milled a further6-8 hours, until an average particle size of 0.6 μm was reached.

The prepared slurry was then poured into a shallow tray and left toevaporate at room temperature for 12 hours. The resulting dried cake ofmaterial was further dried in an oven at 100° C. for 2 hours. The powderwas milled to break up agglomerates.

50 g of the milled powder was then made into a dough. Additives wereprepared by weight of ceramic powder. 2% polyethylene glycol-400(PEG-400) was mixed with 4.5% distilled water, and then added to thepowder and mixed for 2-3 minutes. Following that, 10% Duramax B-1051 and2.5% B-1052 were blended together, and then mixed into the powder,mixing for 3-5 minutes. A further 7-10% distilled water and 1.5% AMP-95was mixed into the powder, and mixed for approximately 10 minutes toform a dough. The dough was kneaded by hand for 1-2 hours before beingleft to age in a sealed plastic bag for 4-6 hours. The dough was kneadedagain for approximately 30 minutes, and then passed through theextrusion die several times to ensure homogeneity. The dough was kneaded5 minutes more before being extruded into tubes and enclosed in tubeholders to dry for 24 hours. The tubes were then sintered for 2 hours at1300° C.

In accordance with a further embodiment of the invention, nopore-forming substance is added to create the pores in the anode tube.Addition of a pore-forming substance creates the risk of changing thesize of the tube when the pores form, and of creating cracks in theelectrolyte layer. Thus, an embodiment according to the invention avoidsthe need to add a pore-forming substance. This embodiment involves firstcreating a fully dense, sintered system (which may be made, for example,in accordance with the process of the embodiment of FIG. 3, or themanufacturing process “Example”); and then reducing the nickel oxide (orother oxide of an electrochemically active substance) that is present inthe anode, to form nickel (or other reduced form of an electrochemicallyactive substance). The reduction may be performed, for example, bypassing a reducing gas (such as hydrogen) through the cell, at theoperating temperature of approximately 800 to 1000° C., after sinteringthe cell in air.

In accordance with another embodiment of the invention, catalysts may beadded to the anode layer to facilitate reformation of a hydrocarbon fuelgas. Instead of spraying such catalysts onto the anode's surface, theymay be added to the anode at the milling stage, and extruded with theanode, to allow in situ catalysis.

An example of a reaction occurring in a solid oxide fuel cell that usesa hydrocarbon fuel gas may be expressed, in a simplified form thatignores partial reactions, as:

$\begin{matrix}{{O_{2 -} + {CH}_{4}}\underset{\{{{Catalyst}\mspace{14mu} 1}\}}{\rightarrow}{{{CO} + H_{2}}\underset{\{{{Catalyst}\mspace{14mu} 2}\}}{\rightarrow}{{CO}_{2} + {H_{2}O}}}} & \left\{ {{Equation}\mspace{14mu} 2} \right\}\end{matrix}$

Here, methane (CH4) is used as the hydrocarbon gas, but otherhydrocarbons may be used. The first half reaction of Equation 2, apartial oxidation, may be catalyzed by CeO₂ (cerium), ruthenium,rhodium, rhenium, or palladium, or other catalysts in accordance withembodiments of the invention. By encouraging the reaction of Equation 2,such catalysts reduce harmful “coking” reactions, such as:

CH₄—>C+2H₂  {Equation 3}

Such “coking” reactions can degrade performance by producing carbondeposits, and may result in a cell's anode lifting away from itselectrolyte layer.

The second half reaction of Equation 2 may be catalyzed, in accordancewith embodiments of the invention, by oxidation catalysts such as:scandia, titania, vanadia, chromium, manganese, iron, cobalt, nickel,zinc, and copper. A catalyst may be selected from amongst such catalystsin order to optimize performance with a particular hydrocarbon fuel. Thesimple oxide of these catalysts may be used, or a pyrochlore orperovskite form.

In accordance with embodiments of the invention, the catalysts describedabove are milled with the NiO/YSZ mixture, in a fashion similar to thatdescribed in steps 301 to 303 above. They can then be extruded alongwith the other ingredients of the anode layer, and used to produce insitu catalysis. Anywhere from 0 to 10% by weight of the catalysts in theanode is preferable, depending on the hydrocarbon fuel with which thecatalyst is being used. About 2% by weight of CeO₂ is preferable for usewith methane fuel. Instead of extruding the catalysts with the anode,they may alternatively be sprayed, co-extruded, or dip-coated onto theanode, in a thin layer.

FIG. 4 shows a graph of chemical gradients in a co-extruded anodesupport of a fuel cell, according to an embodiment of the invention. Inthis embodiment, a process for manufacturing a solid oxide fuel cellinvolves similar steps to those described for FIG. 3 and themanufacturing process “Example,” but the anode is formed by co-extrusionof more than one layer, each layer having different proportions of anelectrochemically active substance and an electrolyte substance. Ananode functions in a fuel cell by providing an electrochemically activesubstance. Typically, in a solid oxide fuel cell, the anode's activesubstance is nickel. If a pure nickel anode were formed on a fuel cell'selectrolyte layer, however, the nickel layer would split away from theelectrolyte layer upon heating, because the layers would have differentthermal characteristics. Thus, in order to match the thermalcharacteristics of the electrolyte layer, it is desirable for higherproportions of electrolyte material to be present in the anode portionsthat are nearer the electrolyte layer. However, it is also desirable forhigher proportions of the electrochemically active substance to be asclose as possible to the hydrogen-containing fuel, to optimizeperformance.

An anode which satisfies both of these constraints thus has opposinggradients of the electrochemically active substance and the electrolytesubstance, from the inner surface of the anode layer to the outersurface of the anode layer. FIG. 4 shows the volume percentages ofnickel 400 (as an example of an electrochemically active substance) andyttria-stabilized zirconia (YSZ) 410 (as an example of an electrolytesubstance), in an anode produced by a co-extrusion of multiple anodelayers, in accordance with an embodiment of the invention. In the layerat the inner surface of the anode the volume percent of nickel 400 ishighest, in order to produce high electrochemical activity, while thevolume percent of YSZ 410 is lowest. But in the layers of the anode atfurther distances from the inner surface, the volume percent of YSZ 410increases, in order to match the thermal characteristics of theelectrolyte layer, while the volume percent of nickel 400 decreases.Solid lines 401 represent the proportions present before sintering; ascan be seen, the layers form a step-like pattern of opposing gradientsof YSZ and Ni. Migration of the substances during sintering may causethe actual proportions of the substances to be smoothed-out somewhat, asrepresented by dashed lines 402. Use of an increasing number of layersin the co-extruded anode thus allows approximation of a smoothly varyinggradient of YSZ/Ni.

In accordance with embodiments of the invention, the proportions ofelectrochemically active substance and electrolyte substance in thecell-supporting anode need not vary as shown in FIG. 4, but may haveother relative distributions of the two substances. For example,different numbers of layers may be co-extruded. In one embodiment, twoanode layers are co-extruded, one of which has a relatively high ratioof electrochemically active substance to electrolyte substance and is ofsmaller diameter, and another of which has a relatively low ratio ofelectrochemically active substance to electrolyte substance and is oflarger diameter.

Alternatively, the anode may have a composition with a uniform ratio ofelectrochemically active substance to electrolyte substance. In thisembodiment, a total volume percentage of about 40-50% nickel in theanode layer is preferable. The anode layer may act to some degree as acurrent collector, as well as a support tube, in such a case, therebyeliminating the need to wrap a current-collecting wire throughout theinside of the fuel cell.

In another embodiment, the anode is formed by co-extruding a thickersupport layer with a thinner active layer. In this case, the supportlayer has a high proportion of YSZ, and a low proportion of NiO (forexample, from 0 to 50 Vol %); and is positioned at the inner surface ofthe anode. The thinner active layer has a higher proportion of NiO (forexample, from 40-45%), and is positioned between the support layer andthe electrolyte. In such a case, current collection may occur throughthe active layer; for example, the active layer may be extruded around acurrent-collecting wire. The thicker support layer may be formed of anelectrolyte substance (such as YSZ), or may instead be formed ofaluminum oxide.

Note that the particular substances (nickel and YSZ) and proportionsshown in FIG. 4 should not be taken to limit the invention to thespecified embodiment; other substances and proportions may be used inaccordance with embodiments of the invention.

FIG. 5 shows a block diagram of a process for manufacturing a solidoxide fuel cell that includes co-extruding a cell-supporting anode, inaccordance with an embodiment of the invention. In a similar fashion tothat described for FIG. 3 and the manufacturing process “Example,” theprocess involves first milling and grinding solid ingredients, in step501. As above, these ingredients preferably include nickel oxide (NiO),which provides the electrochemical activity for the anode, andyttria-doped stabilized zirconia (YSZ). In accordance with oneembodiment of the invention, here called a first example of aco-extrusion process, two mixtures of the ingredients are separatelyformed: one with a relatively high ratio of electrochemically activenickel to electrolyte YSZ, and one with a relatively low ratio of nickelto YSZ. As above, the substances are milled, ground, and analyzed forparticle-size distribution. Alternatively, in a second example of aco-extrusion process, more than two mixtures having differentcompositions of nickel and YSZ are formed, which will subsequently beformed into multiple layers in the anode.

In step 502 of the process of the embodiment of FIG. 5 the anode's solidingredients are formed into a plastic mass for co-extrusion. As above,this is performed by adding a solvent to the mixed solid ingredients,and adding other additives, and aging the mixture. In accordance withthe first example of a co-extrusion process, two separate plastic massesare formed, one from the high-ratio solid mixture and one from thelow-ratio solid mixture. Alternatively, in the second example of aco-extrusion process, more than two plastic masses are formed, that willsubsequently be formed into multiple layers in the anode.

In step 503 of the embodiment of FIG. 5, plastic masses are co-extrudedthrough a die to form an extruded anode tube. In the first example of aco-extrusion process, the high-ratio plastic mass may be extrudedthrough a die of smaller diameter, at the same time that the low-ratioplastic mass is extruded through a die of larger diameter, thatsurrounds the first die. This produces a co-extruded anode tube with twoconcentric layers, one high in electrochemically active substance, theother high in electrolyte substance. Alternatively, the high-ratio andlow-ratio plastic masses are separately extruded, with the high-ratiomass having a smaller diameter than the low-ratio mass; and then theresulting high-ratio extruded tube is fitted inside the resultinglow-ratio extruded tube. This also produces an anode tube withconcentric bi-layers. In the second example of a co-extrusion process,more than two plastic masses are co-extruded, through more than two diesthat surround one other.

In step 504, the co-extruded anode tube is dried and sintered. Theresulting anode, in one embodiment, has a thickness in the range of 300μm to 400 μm. In steps 505 and 506, an electrolyte layer is coated ontothe anode tube, and a cathode layer is coated onto the electrolytelayer, in a similar fashion to that described above.

FIGS. 6A and 6B show an anode-supported tubular solid oxide fuel cell600, in accordance with an embodiment of the invention, that may bemanufactured by the co-extrusion process of FIG. 5. An anode, formed byco-extrusion, is of sufficient thickness to support the fuel cell, andcontains two concentric layers: inner layer 661 contains a higher volumepercentage of an electrochemically active substance, and outer layer 662contains a higher volume percentage of an electrolyte substance.Electrolyte layer 640 is able to be thinner than it would be if it hadto support the fuel cell, and is thus of lower resistance thanconventional electrolyte layers. Cathode layer 620 surrounds theelectrolyte layer 640, and oxidizing gas 625 and fuel gas 665 flow onopposing sides of the tube. Since the anode layer (661 and 662) supportsthe cell, its thickness T3 may represent 50% or more of the totalthickness of the cell (T1 plus T2 plus T3), and electrolyte layer 640may be correspondingly thinner than in a conventional cell.

While the above description has described processes in which acell-supporting anode is formed as the innermost layer of a tubular fuelcell, it is equally possible to produce a cell-supporting anode as theoutermost layer of a tubular fuel cell, in accordance with an embodimentof the invention. In such a case, the process for manufacturing theanode is similar to that described above, except that an electrolytelayer and a cathode layer are coated on the inside of the anode layerafter it has been extruded and sintered. Also, to manufacture such acell by a co-extrusion process, a higher ratio of electrochemicallyactive substance to electrolyte substance would be present at largerdiameters of the anode (rather than smaller). In operation, a fuel gas(containing hydrogen) would be made to flow on the outside of thetubular fuel cell, while an oxidizing gas (containing oxygen) would bemade to flow on the inside of the tubular fuel cell.

Further embodiments of the invention include electrode-supported oxygenpumps and oxygen sensors. Along with solid oxide fuel cells, solid stateoxygen pumps and oxygen sensors are both examples of solid stateelectrochemical cells.

Solid state oxygen pumps are typically used to remove the oxygen gascomponent from a mixture of gases. For example, they enable oxygen gasto be removed from an N₂/O₂ mix, or from an Argon/O₂ mix.

Solid state oxygen sensors generate a voltage that depends on thepartial pressure of oxygen in a gas with which they are in contact, andare commonly used to sense whether a car engine is running “fuel rich”or “fuel lean” by sensing oxygen levels in the car's exhaust.

Oxygen pumps and oxygen sensors, which may be made in accordance withthe embodiment of FIGS. 7A and 7B, both operate in accordance with theNernst equation:

$\begin{matrix}{ɛ = {\frac{RT}{n\; }{\int_{{PO}_{2}^{\prime}}^{{PO}_{2}^{\prime\prime}}{t_{i}\ {{\ln \left( {PO}_{2} \right)}}}}}} & \left\{ {{Equation}\mspace{14mu} 4} \right\}\end{matrix}$

for electromotive force ε, gas constant R, temperature T, n electronstransferred (e.g. 4 for O₂), Faraday's constant

, transport number t_(i), and oxygen partial pressures PO₂′ and PO₂″ onopposite sides of the cell layers. The transport number t_(i) is givenby

$\begin{matrix}{t_{i} = \frac{\sigma_{i}}{\sigma_{i} + \sigma_{e} + \sigma_{h}}} & \left\{ {{Equation}\mspace{14mu} 5} \right\}\end{matrix}$

for an electrolyte having ionic conductivity σ_(i), electronconductivity σ_(e), and hole conductivity σ_(h). For an electrolyte thatis a poor electronic conductor, but a good ionic conductor (i.e. whenσ_(e) and σ_(h) are small compared with σ_(i)), the transport number isapproximately 1 (as follows from Equation 5). This is the case, forexample, when zirconia (a common electrolyte) is used as the electrolytelayer. For such a case, Equation 4 can be approximated as

$\begin{matrix}{ɛ = {\left( \frac{RT}{n\; } \right){\ln \left( \frac{{PO}_{2}^{\prime}}{{PO}_{2}^{\prime\prime}} \right)}}} & \left\{ {{Equation}\mspace{14mu} 6} \right\}\end{matrix}$

In an oxygen pump, an electromotive force is applied across theterminals of the pump, thereby causing oxygen partial pressures PO₂′ andPO₂″ to adjust to the levels specified by the Nernst equation. Thusoxygen ions are pumped through the electrolyte layer; one oxygen partialpressure (for example that of the N₂/O₂ mix 765 in FIGS. 7A and 7B) isreduced, and another oxygen partial pressure (for example that of air725 in FIGS. 7A and 7B) is increased.

Conversely, in an oxygen sensor, a gas containing a known partialpressure of oxygen, PO₂′, (for example air 725 in FIGS. 7A and 7B) isplaced on one side of the sensor, and a gas containing an unknownpartial pressure, PO₂″ (for example N₂/O₂ mix 765 of FIGS. 7A and 7B) isplaced on the other side. The resulting electromotive force ε may bemeasured as a voltage across the electrodes of the sensor.

FIGS. 7A (cross-sectional isometric view) and 7B (cross-sectional view)show an electrode-supported oxygen pump or oxygen sensor according to anembodiment of the invention. In this embodiment, tubular oxygen pump orsensor 700 is mechanically supported by a thick anode layer 760, uponwhich thin electrolyte layer 740 and cathode layer 720 are formed. Asshown in FIG. 7B, anode layer 760 has a greater thickness T₃ than thethicknesses T₁ and T₂ of the electrolyte layer 740 and the cathode layer720. Since the anode layer 760 supports the cell, its thickness T3 mayrepresent 50% or more of the total thickness of the cell (T1 plus T2plus T3), and electrolyte layer 740 may be correspondingly thinner. Inalternative embodiments, pump or sensor 700 is mechanically supported bya thick cathode layer, upon which a thin electrolyte layer and anodelayer are formed; or by both an anode and a cathode, of similar or equalthickness, with a thin electrolyte between. In any of these cases, sincepump or sensor 700 is mechanically supported by an electrode (eitheranode 760, cathode 720, or both), there is no need for a thickenedelectrolyte layer to support the cell. Thus the thickness (l) of theelectrolyte layer 740 may be less than it would be in anelectrolyte-supported cell. As follows from Equation 1, a lowerthickness (l) means that cell 700 has a lower resistance. Oxygen pumpsand sensors according to this embodiment thus have a lower resistancethan electrolyte-supported systems would have. A thinner electrolytealso allows low temperature use, and permits maximum flow of oxygen.

Alternative embodiments of oxygen sensors according to the invention maybe configured to have the gas with an unknown partial pressure of oxygenflow through the center of the tube, while the gas with a known partialpressure of oxygen flows on the outside of the tube; or may be reversed,with the unknown gas flowing on the outside of the tube, and the knowngas flowing through its center. Similarly, alternative embodiments ofoxygen pumps may have the mixed gas flow through the center of the tube,with oxygen pumped outwards; or may have the mixed gas flow on theoutside of the tube, with oxygen being pumped inwards. For both oxygensensors and oxygen pumps, the two alternative configurations differ fromeach other by the relative placement of the anode and cathode (i.e. bywhether the cathode is the innermost layer and the anode outermost, orvice versa).

In one embodiment according to the invention, an electrode-supportedoxygen pump or oxygen sensor is manufactured by an analogous techniqueto that of the embodiment of FIG. 3 and of the manufacturing process“Example.” However, instead of extruding a cell-supporting anode madeof, for example, a mixture of nickel and YSZ, there is extruded aprecious metal mixed with an electrolyte substance, to form an electrodecapable of supporting the sensor or pump. The precious metal mayinclude, for example, platinum, palladium, silver, rhodium, or rhenium;while the electrolyte substance may include, for example, YSZ. Theprecious metal need not be mixed with an electrolyte substance, but mayalternatively be pure. To complete the manufacture of the pump orsensor, a YSZ electrolyte is layered onto the anode, in a similarfashion to that described for the embodiment of FIG. 3 and themanufacturing process “Example”; and then a layer of platinum is sprayedonto the electrolyte layer, to form the cathode.

In another embodiment, an electrode capable of supporting an oxygen pumpor oxygen sensor is manufactured by extruding a porous perovskite, suchas doped LaCoO₃ or doped La[CoFe]O₃.

In both the precious metal and perovskite embodiments, an electrolytelayer is layered around the first electrode, and a second electrode islayered around the electrolyte layer. The second electrode is preferablyformed of the same material as the first electrode.

In one embodiment of an oxygen sensor or pump according to theinvention, the electrolyte layer is made of YSZ, in a similar fashion tothat described for the embodiment of FIG. 3 and the manufacturingprocess “Example.”

In another embodiment, the electrolyte layer is made of a thin layer ofYSZ (or other electrolyte substance) along with a porous support layer,such as an alumina layer.

In another embodiment, the electrolyte layer is made of doped ceriumoxide (CeO₂), which may be doped with calcia or yttria. In this case,the surface of the electrolyte layer may be reduced (for example bypassing a reducing gas over its surface) to form CeO_(2-x), which is anN-type electronic conductor. The reduced layers on the electrolyte maythus play the role of electrodes, and there is no need for furtherseparately layered electrodes to be applied. Alternatively, separateelectrode layers may be applied, without reducing the surface of theelectrolyte. Instead of CeO₂, doped Ln₂O₃, or other lanthanum oxides,may be used; or doped bismuth oxide (Bi₂O₃); yttrium oxide (Y₂O₃); orlead oxides (PbO).

In another embodiment, the electrolyte layer may be made of a porousperovskite, such as doped LaCoO₃ or doped La[CoFe]O₃.

It to be understood that analogous features of oxygen pumps and oxygensensors according to an embodiment of the invention are manufactured inan analogous fashion to that described for the fuel cell embodimentsabove. Layers may be applied by extrusion, co-extrusion, spraying,dipping, coating, or other methods as appropriate to the materials used.

It should also be noted that, for fuel cell, oxygen pump, and oxygensensor embodiments, tubes may alternatively be manufactured in open- orclosed-ended versions (with closed ends being formed, for example, bypinching an open end of an extruded tube).

In further embodiments of fuel cells, oxygen pumps, and oxygen sensorsaccording to the invention, hollow tubes may be manufactured (forexample, by extrusion) that have a non-circular cross-section. In oneembodiment, the tube has a star-shaped cross-section, instead of acircular cross-section, but other cross-sections are possible. Thecross-section used may be adjusted depending upon desired fuel cellpacking density and thermal characteristics. Thus, it should beunderstood that where, above, reference is made to an inner diameter ofa tube, similar considerations apply to an inner surface of a tube (forexample where the tube's cross-section is non-circular).

Although this description has set forth the invention with reference toseveral preferred embodiments, one of ordinary skill in the art willunderstand that one may make various modifications without departingfrom the spirit and the scope of the invention, as set forth in theclaims.

1. A method for manufacturing a solid oxide fuel cell, the methodcomprising: forming a tubular anode comprising an electrolyte substanceand an oxide of an electrochemically active metallic substance without adistinct pore forming substance; sintering the tubular anode; forming anelectrolyte onto the sintered anode; forming a cathode onto theelectrolyte; and after forming the electrolyte and the cathode, reducingthe oxide of the electrochemically active metallic substance in thesintered anode to form pores in the anode.
 2. A method according toclaim 1, wherein sintering the tubular anode comprises: drying thetubular anode; and sintering the tubular anode in air in a furnacehaving a furnace temperature ramp rate of approximately 0.5° C. perminute, up to approximately 500° C., followed by a ramp rate ofapproximately 3° C. per minute up to approximately 1300° C., and a dwelltime of approximately 2 hours for sintering.
 3. A method according toclaim 1, wherein the electrolyte substance of the anode includesstabilized zirconia and wherein the oxide of the electrochemicallyactive metallic substance includes at least one of a nickel oxide, aplatinum oxide, a palladium oxide, and a cobalt oxide.
 4. A methodaccording to claim 1, wherein forming and sintering the tubular anodecomprises: forming a plastic mass comprising a mixture of theelectrolyte substance and the oxide of the electrochemically activemetallic substance; extruding the plastic mass through a die to form anextruded tube; and sintering the extruded tube.
 5. A method according toclaim 1, wherein the anode comprises more than one anode layer, eachlayer having a different composition.
 6. A method according to claim 5,wherein there are two anode layers.
 7. A method according to claim 5,wherein there are more than two anode layers.
 8. A method according toclaim 5, wherein each of the anode layers comprises a ratio ofelectrochemically active metallic substance to electrolyte substance,and wherein such ratios are higher for layers that are layered furtherfrom a surface of the anode that contacts a fuel gas than for layersthat are layered closer to the fuel gas.
 9. A method according to claim5, wherein forming and sintering the tubular anode comprises: formingfirst and second plastic masses, each plastic mass comprising a mixtureof the electrolyte substance and the oxide of the electrochemicallyactive metallic substance, the first plastic mass having a higherrelative content ratio of oxide to electrolyte substance, and the secondplastic mass having a lower relative content ratio of oxide toelectrolyte substance; extruding the first plastic mass through a die toform a first extruded tube; extruding the second plastic mass through adie to form a second extruded tube; fitting the first extruded tubeinside the second extruded tube to form a combined tube; and sinteringthe combined tube.
 10. A method according to claim 9, wherein eachplastic mass comprises a mixture of stabilized zirconia and nickeloxide, the first plastic mass having a higher relative content ratio ofnickel oxide to stabilized zirconia, and the second plastic mass havinga lower relative content ratio of nickel oxide to stabilized zirconia.11. A method according to claim 5, wherein forming and sintering thetubular anode comprises: co-extruding more than one anode layer to forma co-extruded tube; and sintering the co-extruded tube.
 12. A methodaccording to claim 5, wherein the more than one anode layers comprise athicker support layer for contact with a fuel gas and a thinner activelayer.
 13. A method according to claim 12, wherein the support layercomprises a higher ratio of stabilized zirconia to nickel, and whereinthe active layer comprises a lower such ratio.
 14. A method according toclaim 12, wherein the support layer comprises from 0% to 50% nickel byvolume.
 15. A method according to claim 12, wherein the active layercomprises from 40% to 45% nickel by volume.
 16. A method according toclaim 12, wherein the active layer comprises an embeddedcurrent-collecting wire.
 17. A method according to claim 16, wherein theactive layer is extruded around the current-collecting wire.
 18. Amethod according to claim 12, wherein the support layer comprisesaluminum oxide.
 19. A method according to claim 1, wherein the anodefurther includes a catalyst material.
 20. A method according to claim19, wherein the catalyst material includes at least one of: CeO₂,ruthenium, rhodium, rhenium, palladium, scandia, titania, vanadia,chromium, manganese, iron, cobalt, nickel, zinc, and copper.
 21. Amethod according to claim 19, wherein the catalyst material includesCeO₂ in a proportion of between 1% and 3% by weight.
 22. A methodaccording to claim 19, wherein forming the anode comprises milling thecatalyst with the oxide of the electrochemically active substance.
 23. Amethod according to claim 1, wherein reducing the oxide of theelectrochemically active metallic substance in the sintered anode toform pores comprises flowing a reducing gas over a surface of thesintered anode.
 24. A method according to claim 1, wherein reducing theoxide of the electrochemically active metallic substance in the sinteredanode to form pores comprises flowing hydrogen gas over a surface of thesintered anode at a temperature between approximately 800° C. and 1000°C.
 25. A method according to claim 1, wherein the anode comprises asubstantially uniform ratio of electrochemically active metallicsubstance to electrolyte substance.
 26. A method according to claim 1,wherein the anode comprises a volume percentage of nickel of between 40%and 50%.
 27. A method according to claim 1, wherein a thickness of theanode comprises over 50% of a total thickness of the anode, theelectrolyte, and the cathode.
 28. A method according to claim 1, whereinthe anode has a thickness of at least 300 μm.
 29. A method according toclaim 1, wherein the anode has a circular cross-section.
 30. A methodaccording to claim 1, wherein the anode has a non-circularcross-section.
 31. A method according to claim 1, wherein theelectrolyte formed onto the sintered anode comprises stabilizedzirconia.
 32. A method according to claim 1, wherein forming theelectrolyte onto the sintered anode comprises at least one of: sprayingthe electrolyte onto the sintered anode; and dip-coating the electrolyteonto the sintered anode.
 33. A method according to claim 1, wherein thecathode comprises a strontia-doped lanthanum manganite.
 34. A methodaccording to claim 1, wherein the cathode comprises at least one of:gadolinium manganate; and a cobaltate.
 35. A method according to claim1, wherein the cathode comprises more than one cathode layer, eachcathode layer having a different composition.
 36. A method according toclaim 35, wherein there are two cathode layers.
 37. A method accordingto claim 36, wherein the two cathode layers comprise: an inner cathodelayer comprising a mixture, 50/50 wt % of La_(0.80)Sr_(0.20)MnO₃(Rhodia, 99.9% pure) with 8 mol % YSZ (Tosoh); and an outer cathodelayer comprising substantially only La_(0.80)Sr_(0.20)MnO₃ (Rhodia,99.9% pure).
 38. A method according to claim 35, wherein there are morethan two cathode layers.
 39. A method according to claim 1, whereinforming the cathode onto the electrolyte comprises spraying at least onecathode layer onto the electrolyte.